Some examples are sulphonic acid (US sulfonic), a member of a group of organic acids containing the group —SO2OH; phosphonic acid, a crystalline dibasic acid of formula HPO(OH)2; propionic acid (Greek pro, before, plus piōn, fat, it being the first member of the fatty acid series to form fats), a liquid organic acid produced in some forms of fermentation and used for inhibiting the growth of mould in bread; arachidonic acid, a polyunsaturated fatty acid present in animal fats, important in metabolism.